Cerebral vasospasm is not well understood leading to indeterminate diagnosis and delayed treatment following aneurysmal subarachnoid hemorrhage (A-SAH). There is poor clinical outcome associated with vasospasm due to permanent neurologic deficit, stroke and death. Stroke has been detected in up to 81% of A-SAH patients, affecting 21,000-33,000 persons each year in the United States. As many as 46% have long-term cognitive impairment, affecting functional status and quality of life. Thereby, this disorder is also associated with a substantial burden on public health care costs and resources, mostly related to hospitalization, long-term care and indirect economic costs of disability. Our long-term goal is to improve clinical outcomes of patients following A-SAH by implementing CT perfusion (CTP) as a means to 1) provide earlier and more accurate diagnosis of vasospasm, and 2) guide effective treatment strategies. Our hypothesis for this research proposal is that appropriate use of CTP will improve the clinical outcomes of A-SAH patients by providing earlier and more accurate diagnosis of vasospasm. Using a prospective cohort study design, A-SAH patients will be monitored according to their symptoms, transcranial Doppler ultrasound (TCD), CT angiography (CTA), CTP and digital subtraction angiograpghy (DSA) exams. The primary objective is to determine the performance of CTP as a diagnostic tool in the evaluation of vasospasm, by these specific aims: 1) To determine the sensitivity, specificity, positive and negative predictive values of CTP, 2) to compare sensitivity and specificity of CTP with symptoms, TCD, CTA and DSA, and 3) to identify the combination of modalities that provides the earliest diagnosis. The secondary objective is to evaluate the impact of CTP for diagnosis of vasospasm, with these specific aims: 4) To assess the effect of CTP on diagnostic certainty and management decision making, 5) to determine the effect of CTP on clinical outcomes, and 6) to perform cost-effectiveness analysis. A strong mentoring program at my institution with a well-structure educational curriculum at the Harvard School of Public Health and Weill Graduate School of Medical Sciences of Cornell University will greatly enhance my clinical research skills and build on my prior research experience to develop into an independent clinical investigator in this field.